There are conventional devices that carry out multi-step analytical procedures in an automated or semi-automated fashion. For example, microbiological analytical systems currently carry out automated antimicrobic susceptibility testing procedures using both photometric and fluorometric detection methods. The MicroScan Division of Baxter Healthcare Corporation sells a device of this type under the trade designation "Walk-Away." Armes et al. U.S. Pat. No. 4,676,951 and Hanaway U.S. Pat. Nos. 4,643,879 and 4,681,741 describe certain features the Walk-Away system.
Prior commercial embodiments of the Walk-Away system analyze trays carrying microbiologic specimens. The system includes an enclosed incubation chamber for the specimens. The system adds reagents to the specimens and analyzes them.
Prior commercial embodiments of the Walk-Away system and systems of the same type use stepper motors to operate many of the moving mechanical components. The coils of a stepper motor are sequentially energized to rotate the associate rotor in discrete, predetermined "steps." Motor operation can be accurately luminating controlled and programmed by counting the steps.
Typically, current to a conventional stepper motor coil is "chopped" while the coil operates in its energized phased. This means that current flow through the coil is sequentially enabled and not enabled by an associated switching mechanism, using, for example, a field effect transistor. This modulates the current, keeping it from rising above the nominal motor current. In conventional operation, an associated circuit (called a "flyback" circuit) dissipates energy from the coil during the chopping mode. The flyback circuit also dissipates energy when the coil switches from its energized phase to its de-energized phase.
When the coil is energized and operates in its chopping mode, the flyback circuit should ideally dissipate energy slowly to provide sustained, steady power to the motor. When the coil is switched to its de-energized phase, the flyback circuit should ideally dissipate energy quickly to provide a smooth and quick transition to the successor coil. Conventional flyback circuits seek to compromise these two competing operating objectives. As a result, neither objective is fully met.